<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title></title>
</head>

<body>
<form id="myform"  method="post" action="#">
    <select name="grade" id="grade">
        <option value="1">初级</option>
        <option value="2">中级</option>
        <option value="3">高级</option>
    </select><br><br>
    <button id="btn" name="btn" type="button" >移出选项</button>
</form>
<script>


var form = document.getElementById("myform");
var grade = form.elements["grade"];                       
var btn = form.elements["btn"]; 

btn.onclick = function(){
	for(var i=0, len=grade.options.length; i < len; i++){
		grade.remove(0);
	}
	this.disabled = true;
	this.innerHTML = "已全部删除";
	return false;
}
                

</script>
</body>
</html>
